I'm new to photography and just starting to take photo in RAW format. I haven't found a difference in quality of color adjustment editing between ViewNX (free software coming with my Nikon d5100) and Picasa, perhaps because of my inattentiveness.
So I've decided to ask here what is the best free software to get most from RAW (Nikon NEF) photo editing?

4 Answers
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For the Mac, the best editor that will get the most (but is not exactly easy to use) is the Raw Photo Pro processor. It can do pretty much anything to maximize the raw potential. But if I may, I think that's going down the wrong path. You can get a little extra quality from processing the raw differently (I'm intentionally excluding post-raw conversion editing) but far more quality from how you setup the shot. Change in lighting, filters, angle, exposure, etc. can dramatically improve a picture more than subtle raw conversion improvements.

Even still, once a photo has been converted there is a world of changes you can do to an image that has far more ability to change perceived quality than what is done through raw conversion.

"Best" depends on what you're looking for in post-processing software, but a few of the more popular open source packages you could look at using would be:

The GIMP with dcraw. The GIMP is the open-source alternative to Photoshop, and has a very deep and sophisticated feature set, with quite a bit more control than you could find in Picasa. Tutorials are plentiful out on the interwebz. dcraw adds raw capability to the Gimp, so you do need to use them in conjunction.

If, however, you'd prefer something more workflow-oriented that looks more like a RAW conversion tool such as Lightroom, vs. Photoshop, then three open source packages to consider are RAWTherapee, darktable (no Windows version, only Linux/OSX), and photivo. These types of editors are non-destructive (i.e., the original RAW file is left intact, and edits are saved separately. Upon export, the instructions in the edits are then applied to the original file--you always have access to all your original image data), and are generally easier to learn than a tool like the Gimp or Photoshop. They may be more feature-limited, but they have much more editing capability than Picasa or iPhoto.

RawTherapee is quite good and "feature packed" and works on multiple platforms.One of its strengths has always been its excellent highlight recovery. In addition, it added last year two demosaicing algorithms optimized for noisy images. I tested them and they a markedly superior to anything else out there.